Gas turbine engines are known, and when utilized on an airplane, typically have a fan that delivers air both into a bypass duct defined inwardly of a nacelle and into a core duct the leads to a compressor. The air is compressed in the compressor and delivered into a combustor where it is mixed with fuel and ignited. Products of this combustor pass downstream over turbine rotors driving them to rotate. The turbine rotors, in turn, drive the fan and the compressor.
Historically, a fan drive turbine drove both the fan and a low pressure compressor through a direct drive connection such that all of the fan drive turbine, the fan, and the low pressure compressor rotated at the same angular velocity. By tying the speed of the fan to the fan drive turbine, this not only limited the speed of the fan drive turbine, but also was a design restriction on the diameter and speed of the fan. For many reasons, it would be desirable for the fan to rotate at a slower speed, thereby enabling it to be radially larger.
More recently, a gear reduction has been provided between the fan drive turbine and the fan. The gear reduction has allowed the fan diameter to increase dramatically. With the increase in fan's diameter, a bypass ratio, or volume of air delivered into the bypass duct compared to the volume of air delivered into the core duct that leads to the compressor, has also increased. As a result of the increase in the bypass ratio, negative aerodynamic effects have been identified in the overall propulsion system that includes the nacelle and the engine. Accordingly, what is needed is an improved propulsion system that does not experience these negative aerodynamic effects.